The present invention relates to an atomiization bowl and to a rotating electrostatic sprayer for atomizing, a coating product, equipped with such a bowl.
It is known to equip a rotating coating-product sprayer with a bowl or bell fast with the rotor of a turbine for drive in rotation, in order to form a cloud of atomized coating product, this cloud being entrained by an electrostatic field and possibly by a flow of air, in the direction of an object to be coated. Essentially, two types of atomization bowls exist, namely atomization bowls made of an electrically conducting material and atomization bowls made of an electrically insulating material.
Bowls made of an insulating material do not allow the coating product to be electrostatically charged, with the result that ancillary means must be provided for charging the coating product through luminous or corona discharges. The charge of the coating product obtained is less efficient, with the result that the effect of the electrostatic field on the coating particles is lesser, the yield of deposit obtained being relatively low.
Bowls made of a conducting material allow the coating product to be electrostatically charged when it flows in contact with the surfaces of the bowl which is taken to high voltage by any appropriate means. These bowls generally present the drawback of constituting high electrical capacitances, potentially dangerous for a user and capable of creating electric arcs likely to set off a fire in the explosive atmosphere of a coating booth.
Furthermore, the electrostatic coating equipment and their peripherals undergo advanced safety tests in order to be sure that they are not likely to provoke an electrical discharge of which the energy would be greater than a predetermined threshold value. Certain standards provide, for example, that the breakdown or discharge energy between a part of the sprayer taken to high voltage and a sphere of small radius disposed at a distance of the order of a centimeter, must be less than 0.24 mJ. Such a value cannot be respected with a known bowl made of electrically conducting material as the edges and plane surfaces that it comprises, in particular near the edge for discharge of the coating product, are capable of producing discharges of much higher energy.
It is a particular object of the invention to overcome these drawbacks, by proposing an atomization bowl allowing a charge of the coating product by contact and for which the discharge or xe2x80x9cbreakdownxe2x80x9d energy is maintained at a level lower than that imposed by the most restrictive standards.
To that end, the invention relates to an atomization bowl for a rotating electrostatic coating-product sprayer, said bowl comprising an end shield for distributing the product provided to be taken to high voltage when the sprayer is operating, so as to charge the product electrostatically by contact, characterized in that the end shield is electrically connected to at least one current discharge element through corona discharge from one end in the form of a tapered point, this end in the form of a tapered point being arranged in the vicinity of an outer surface of the bowl.
Thanks to the invention, the end shield, which constitutes the central part of the bowl, allows a charge by contact of the coating product during flow. The discharge element or elements create privileged paths of corona discharges between the end shield and the outside of the bowl, with the result that no accumulation of energy is possible at a level such that the breakdown energy would exceed the admissible values.
In addition, taking the modelizatioin of an earthed object which approaches the bowl, the approach of the object leads to an increase in the corona discharge Current translated by a drift of the current consumed, this drift being able to be detected by a high voltage generator used for supplying the end shield of the bowl with high voltage. In effect, known generators, such as in particular those of Applicants"" patent applcation EP-A-0 219 409, are capable of reacting to an abnornal variation in the current. In this way, the variations in current generated by the discharge element or elements when an earthed object approaches, are detected by the generator sufficiently early for the safety function thereof to cancel the electrical supply before a potentially dangerous discharge of energy is possible.
According to a first advantageous aspect of the invention, the bowl comprises a plurality of discharge elements regularly distributed around its axis of rotation. This plurality of discharge elements makes it possible, when the bowl is switched on, to obtain the desired effect with a symmetry of revolution about the axis of rotation of the bowl, including in the absence of rotation. In addition, this distribution avoids a dynamic lack of balance of the bowl, i.e. an unbalance non-admissible at the speeds of rotation in question.
According to a first, particularly advantageous embodiment of the invention, the end shield is bordered, on its outer periphery, by a substantially annular piece made of a material which is electrically more insulating than the material of the end shield and forming an atomization edge of the bowl and the current discharge element is formed by an insert made of a material which is electrically more conducting than the annular piece and disposed in this annular piece, in electrical contact with the end shield, this insert extending up to the vicinity of an outer surface of the annular piece. The annular piece located on the periphery of the end shield avoids the risks of electrical breakdown between the plane surfaces taken to high voltage and an adjacent earthed object. The inserts, which are advantageously, but not necessarily metallic, constitute particularly effective discharge elements through the insulating or semi-conducting layer made by the annular piece around the end shield.
According to another advantageous aspect of the invention, the insert is partially received in a bore made in the conducting end shield. This particularly simple construction ensures an efficient electrical contact between the end shield and the insert, while guaranteeing its positioning with respect to the other components of the bowl.
According to another advantageous aspect of the invention, and in the case of a bowl comprising a deflector of the flow of coating product disposed opposite a so-called front surface of the end shield, the deflector bears at least one discharge element extending up to an outer, so-called front surface, of the deflector. The presence of this discharge element in the deflector also makes it possible to avoid the accumulation of electrostatic charges in this deflector and in the coating product in the course of flow around this deflector. The discharge element is advantageously an insert disposed along the axis of rotation of the bowl.
According to another advantageous aspect of the invention, the insert or inserts is or are in needle-form and provided with at least one end in the form of a point, this end being flush with the outer surface of the annular piece or of the deflector. In that case, at least one insert is advantageously provided to bear a flange adapted to cooperate with an inner shoulder of a housing for receiving the insert.
According to an advantageous variant embodiment of the invention, at least one insert is provided with two ends in the form of a point, these ends being flush with two outer surfaces of the annular piece, these outer surfaces being oriented respectively towards the axis of rotation and towards the outside of the bowl. This particular arrangement makes it possible to limit the value of the discharge energy both towards the central zone of the bowl and towards the outside.
According to another advantageous aspect of the invention, the end of the insert close to the outer surface of the bowl is disposed in the vicinity of the atomization edge. This arrangement is particularly advantageous, as it is near the atomization edge that there is more risk of encountering an earthed obstacle, such as a trap door or an unclosed door on the bodywork of an automobile in the course of being coated.
According to another, particularly advantageous embodiment of the invention, the discharge element is formed by a point fashioned on the outer surface of the end shield. This aspect of the invention makes it possible to produce an entirely conducting bowl, particularly efficient for the charge of the coating product by contact, while avoiding the above-mentioned problems of discharge with potentially dangerous levels of energy. In effect, the points formed by the discharge elements are supplied with high voltage via the mass of the end shield and constitute as many privileged points of discharge through corona discharge. The point or points are advantageously formed in one piece with the end shield.
The invention also relates to an electrostatic sprayer of coating product which comprises a bowl as described hereinabove and means for electrically connecting the end shield of this bowl to a high voltage generator, this generator being adapted to detect variations in the corona discharge current transiting through the discharge element or elements of the bowl. This sprayer complies with the most demanding safety standards, which corresponds to very high operational safety.